1. Field of the Invention
The present invention relates to a hot press for producing a multi-layer substrate made of printed circuit boards, and ,more particularly, to a hot press suitable for forming a high quality multi-layer substrate and reducing a cost thereof.
2. Description of the Prior Art
With the demand for multi-layer substrate having a higher density, there are increased demands for ensuring against voids between the layers, displacement between the layers, as well as a demand warpage or twist of the substrate, and for final dimensional precision. Voids are formed by gas, residing in layers which gas is contained in bubble or moisture in the atmosphere when a material for the multi-layer substrate is exposed in the atmosphere or volatile composition contained in an adhesive resin. In order to prevent the generation of the voids, a material for a multi-layer substrate including a resin, called prepreg for adhesion, is heated and pressurized to expell the voids from its marginal portion together with the resin flow. For this purpose, Japanese Patent Unexamined Publication No. 62-23722 shows a technique in which a central portion of the heating plate is made of material having a high thermal expansion coefficient and the heating plate is heated to thereby increase the pressure of the central portion thereof On the other hand, Japanese Patent Unexamined Publication No. 62-56140 discloses a technique in which, in order to remove voids and to enhance a dimensional precision in thickness of products after the press process, a multi-layer substrate is moulded under a reduced pressure for 150 to 500 seconds by controlling the resin amount and the resin flow of the resin-impregnated substrate. Furthermore, Japanese Patent Unexamined Publication No. 62-68748 shows another technique in which a surrounding wall for preventing the adhesive resin molten by heat from overflowing is provided for the substrate under the pressurized condition, thereby crushing and removing the bubble contained in the substrate.
Japanese Patent Unexamined Publication No. 61-295038 shows a technique in which the heating operation and the pressurizing operation are controlled on a theory such a theory that the warpage or twist is due to non-uniformity in resin flow dependence upon a thermal hysteresis and a pressure timing hysteresis of the adhesive resin.
The displacement between the layers not only cause adverse affects in final configuration or parallelism but also damage or breakage of circuit wires or conductors. Japanese Patent Unexamined Publication No. 59-188999 shows a method for overcoming this problem, wherein an auxiliary ram is used to maintain the parallelism in a satisfactory condition against the non-uniformity in dimension of thickness of the material of the multi-layer substrate. In the same manner, Japanese Patent Examined Publication No. 61-54580 proposes a method in which a metallic plate with a displacement preventing portion located along its marginal portion is used.
In order to simultaneously overcome the problems of the void, the displacement, and the dimensional precision in final configuration, Japanese Patent Unexamined Publication No. 57-118698 shows a method in which a space between the thermal plates are maintained under the reduced pressure with application of heat and pressure, and Japanese Patent Unexamined Publication No. 62-156931 shows a method in which the internal portion of the substrate is covered by a film or a sheet to be maintained under a reduced pressure condition, and then the substrate is heated by heating plates. Moreover, Japanese Patent Unexamined Publications Nos. 61-43543 and 61-43565 show a method in which, a high pressure vessel called autoclave is used wherein the material of the multi-layer substrate is covered by a film or sheets, with an interior of the film or sheet being maintained under a reduced pressure condition and, thereafter, the material is heated under pressure with nitrogen gas or CO.sub.2 gas within the high pressure vessel. Also, instead of coverage by the film or sheet, Japanese Patent Unexamined Publications Nos. 61-290036 and 61-293836 show a method in which a box-shaped vessel called pressure chamber, whose top surface is movable up and down, is used. Furthermore, Japanese Utility Model Unexamined Publication 61-185598 shows a method in which an entire press for applying heat and pressure is provided in a vacuum heater.
With high density multi-layer substrate, a width of wires or circuit conductors and a space between the circuit conductors are narrow, a space of lands is small, and through-holes are formed after the formation of the multi-layers. Accordingly, there is a demand for the substrate in which void does not occur during he heat and pressure process; a displacement of the layers is small; there is small warpage or twist; and an expansion or contraction is suppressed for the final dimensional precision as much as possible. At the same time, a breakage of the circuit conductors must be avoided. In order to eliminate or suppress the causes of failure, although the causes may depend upon the material of the adhesive resin interposed between the layers, it is desired to reduce the heating temperature and pressure as much as possible within suitable ranges. In the foregoing prior art, although the pressurizing operation exceeding 50 kg/cm.sup.2 is carried out as disclosed in Japanese Patent Unexamined Publications Nos. 61-295038 and 62-23733, it has been found that a void does not occur even under the pressure below 20 kg/cm.sup. 2 if the suitable temperature condition is selected. Also, it is desired that the atmospheric air and moisture between the layers of the multi-layer substrates is removed once under the reduced pressure condition, and further the preheating process and the prepressurizing process with the thermal plates are carried out under the reduced pressure condition, thereby removing the volatile composition of the adhesive resin; and that thereafter, the environmental space is heated under pressure with a prescribed gas such as, for example, nitrogen gas or CO.sub.2, so that an external force is applied to a peripheral or marginal portion of the multi-layer resin to perform the final production with the thermal plates while preventing the displacement of the layers and the overflow of the adhesive resin.
The prior art document, substantially meeting the foregoing demands, is Japanese patent Unexamined Publication 62-156931. However, this conventional technique suffers from the following disadvantages. Namely, it is difficult to finish the final configuration and dimensional precision within predetermined ranges since this publication does not relate to a heat and pressure process with the thermal plates. Also, this system requires a long time for evacuating the high pressure vessel or heating the vessel. Additionally, the film or sheet for covering the material of the multi-layer substrate and the air pressure chamber are expensive. Also, mounting and removal of the vessel is time-consuming, resulting in low working efficiency. Finally, the entire apparatus is expensive.